I've only seen it once that I can think of, and I don't remember where. It might've been an example when they added or explained that predicate. I recall some

I've only seen it once that I can think of, and I don't remember where. It might've been an example when they added or explained that predicate. I recall something like (Column1 = Column2) IS NOT UNKNOWN, but I don't know why you wouldn't use Column1 IS NOT NULL AND Column2 IS NOT NULL instead. I guess it might save a bit of rewriting, but it still seems pretty narrow. It's really not useful unless you're talking about the value of a boolean expression or the underlying concepts of SQL, and most RDBMSs don't let you manipulate that directly with DML (MySQL is the first one I've seen that let you do it, and you just taught me that was the case). It's somewhat hidden because of that.